Photopolymerizable resist materials are known, for example, from U.S. Pat. No. 3,469,982 which describes a film resist with a sandwich structure in the form of a photopolymerizable layer between a cover sheet and a temporary support. This film can be laminated onto a copper base, exposed imagewise and developed, whereby a resist layer is formed. The copper panel with the resist image can be selectively etched, electroplated, or treated with solder to form a printed circuit board.
Prior art photopolymerizable compositions often contain small amounts of a leuco dye which upon photoimaging is converted to its colored form. This color in the exposed portions of the resist is referred to as the printout image. The printout image enhances the visibility of the exposed areas and facilitates inspection for imperfections in the image both prior to and after development.
Photoimaging compositions useful for forming printout images are known. Yamada et al. U.S. Pat. No. 3,476,562 and Lewis et al. U.S. Pat. No. 3,769,023 disclose printout image formation using compositions containing haloorganic compounds. However the images formed are relatively poor.
U.S. Pat. No. 4,065,315 to Yamazaki et al. discloses the use of halogenated aliphatic amides to enhance the printout image of compositions containing a variety of dye bases, while Reardon in U.S. Pat. No. 4,343,885 discloses the use of fluoran colorformers with a variety of halogenated materials as the image forming system. In U.S. Pat. 4,552,830 Reardon and Lipson disclose the use of aliphatic carbonylic halides as activators for color formation by leuco dyes, with the preferred compounds being halogenated amides of dicarboxylic acids.
Brominated sulfones such as tribromomethyl phenyl sulfone (BMPS) have been reported as printout image enhancers with leuco dyes as disclosed in Japanese examined Patent 3-68,376.
Likewise hexaarylbiimidazoles (HABIs) have long been known to form radicals which can oxidize triarylmethane dyes to their colored form as is disclosed in U.S. Pat. No. 3,445,234. The use of HABIs and BMPS together as printout image enhancers is disclosed by Fujikura et al. in U.S. Pat. No. 5,030,548.
U.S. Pat. No. 4,634,657 discloses the use of leuco dyes along with substituted 1,2-dibromoethanes of the formula: ##STR1## in which R.sub.3 and R.sub.4 are independently hydrogen, chlorine or fluorine. While these compositions form high contrast images, they are rather volatile so that image instability can sometimes occur in films even at room temperature. Polyhalogenated materials such as these and their polyhalogenated byproducts are also undesirable from an environmental standpoint.
Other compositions such as those based on tribromomethylphenyl sulfone or lophine dimers tend to be thermally unstable when heated to higher temperatures for short periods or upon extended storage either in solution or in a film. The instability can be manifested by premature color formation in solution or in a film. Thermal instability can also be manifested by a lowering of the temperature for onset of polymerization of the monomer system which can be a problem if the composition is exposed to high temperatures during film manufacture or during process of use.
Many printout systems are useful only in the presence of acid, or with specific monomers.
Printout enhancers that produce a high contrast image when used in small amounts are also desirable for environmental reasons as regulation on the use of halogenated materials become more stringent. Thus, polyhalogenated materials that must either be used in large amounts to produce high contrast images, or materials like halogenated 1,2-dibromoethanes which produce polyhalogenated byproducts upon reaction, are less desirable as ingredients. Materials which can be used to produce films by melt extrusion instead of solvent coating likewise are desirable from an environmental standpoint since the melt extrusion process lowers air emissions and eliminates residual solvents in the film.
Thus, materials that greatly enhance printout images, without seriously lowering the thermal stability of the film composition are highly desirable to enhance the shelf life of photoimageable film compositions under ambient or elevated temperature conditions and for the preparation of films with a thermal history, such as by melt coating.